This invention relates to the fabrication of gallium-arsenide semiconductor devices having low resistance contacts and, more particularly, to the construction of a thermally-stable contact containing a small amount of indium to yield a low resistance path to electric current, the contract including an additional element for forming thermally stable intermetallic compounds with indium and/or a single solid phase.
Devices fabricated of semiconductor material frequently employ low-restance contacts to provide an electrically conducting path between the semiconductor material and electric circuits external to the devices. A field-effect transistor (F.E.T.) is a common example of a semiconductor device having low-resistance contacts. The low-resistance contacts are of the form known as ohmic contacts and are used in the formation of the source and the drain terminals of the transistor. The low-resistance contact forms a path to the semoconductor device, and allows electrical current to flow into or out of the semiconductor material without significant voltage drop.
A semiconductor material of particular interest herein is gallium arsenide (GaAs). Low resistance contacts to GaAs are essential for the fabrication of many high performance optical, microwave and logic devices. A commonly used low-resistance contact to n-type GaAs is composed of an alloyed gold-nickel-germanium (Au-Ni-Ge) system which produces contact resistance in the range of 0.2-1.0 ohm-millimeters. Other materials frequently used in the construction of non-ohmic (Schottky) contacts of GaAs are tungsten silicide and layers of gold, titanium, and platinum.
In the manufacture and packaging of a semiconductor device, such as the foregoing FET, after formation of low-resistance contacts and non-ohmic (Schottky) gates, the devices are required to withstand an elevated temperature, typically 400.degree. Centigrade, for periods of a few minutes to several hours. Such heating over a sustained interval of time increases the resistance. For example, the resistance of a contact of Au-Ni-Ge has been observed to increase by a factor of three during annealing of the contact for five hours at 400.degree. C.
In the construction of a low-resistance contact, the resistance is optimized by appropriate selection of material composition and by the cleaning of the substrate prior to the deposition of the material used in the low-resistance contact. While presently available techniques may be employed to provide low-resistance contacts with acceptably low values of resistance, the above-noted problem of increase in resistance with prolonged heating limits the application of currently used low-resistance contacts.